[PATCH] D50564: Add support for SEH unwinding on Windows.

[Kristina Brooks via Phabricator via llvm-commits]

kristina added inline comments.


================
Comment at: src/UnwindCursor.hpp:1801
+  if (pc != getLastPC()) {
+    UNWIND_INFO *xdata = reinterpret_cast<UNWIND_INFO *>(base + unwindEntry->UnwindData);
+    if (xdata->Flags & (UNW_FLAG_EHANDLER|UNW_FLAG_UHANDLER)) {
----------------
mstorsjo wrote:
> I can't say I understand all of this yet, but I'm slowly getting there, and I'm trying to compare this to what libgcc does.
> 
> libgcc doesn't use any definition of UNWIND_INFO and doesn't need to do the equivalent of `getInfoFromSEH`, used by `step()`, anywhere. `unw_step()` is used in `_Unwind_ForcedUnwind`, which in libgcc is implemented using `RaiseException (STATUS_GCC_FORCED, ...`.
> 
> I guess if you happen to have all of the `unw_step` API available, it's easier to just do it like this, in custom code without relying on the NTDLL functions for it, while the libgcc version relies more on the NTDLL API.
This primarily deals with the SEH exceptions re-purposed as a C++ exception mechanism on x86_64 (if I understood this right), it's possible to set a custom filter using a runtime call so I suspect GCC does that or defines a translation function (also via a runtime call) which acts as a filter for "true" SEH exceptions behind the scenes deep within the runtime. Typially "true" SEH exceptions don't, outside of runtime magic, play nicely with C++ exceptions, with the `__C_specific_handler` ones being a completely different paradigm that falls far outside the scope of libunwind (those ones being the "true"/explicit SEH exceptions).

(Don't take my word for it, it's been a while and I only implemented the "true" variation for 64-bit Linux by reserving some RT signals and using that to invoke additional runtime glue that would then do the unwinding, completely ignoring DWARF since CFI exceptions and SEH exceptions really don't mix especially on platforms that are not Windows-like)


Repository:
  rUNW libunwind

https://reviews.llvm.org/D50564